[hpsdr] VHF/UHF downconverter project for HPSDR

[Phil Harman]

Hi Ante,

I also have been working on a variant of your B1 solution.

When initially working on the Cyclops spectrum analyser I tested an Si570
for the second LO (1.126GHz). Whilst this was fine in terms of spectral
purity it did drift quite badly with temperature.  For that reason I used
a conventional VCO + PLL system.

More recently, with Cyclops being re-started, I've been evaluating the
ADL4350 synthesiser chip. This chip contains a 2.2 - 4.4GHz VCO,
integer/fractional N PLL and output dividers.  It will provide an output
between 137MHz and 4.4GHz at +5dBm.  Whilst the close in phase noise above
2GHz is not particularly good, in terms of parts count and performance/$
its hard to beat.

Using similar building blocks to Joe I've built a simple down converter
using Mercury at 60MHz as the IF. Using this IF simplifies the design of
the input filters.

I'm also experimenting with using multiple IFs so that either a
combination of LO and IF could be found that avoids any images and/or
subtracting the image signal(s) from the wanted signal (when used as a
spectrum analyser). Early tests look very encouraging.

The use of multiple LO/IF combinations to identify images is not a new
technique. I recall one of the first spectrum analysers I used over 30
years ago had a button you could push in order to determine which of the
signals displayed where images. It was up to the operator to do the final,
visual, signal processing!

With the speed of modern PCs and FFTs we should be able to automate this
process.

If there is any interest I'll write this up for the Wiki.

73 Phil...VK6APH


> ***** High Performance Software Defined Radio Discussion List *****
>
> Hi Ante,
>
> I happen to use a variation of your B1 "solution" on 70 cm EME with
> good results with polarization diversity using an HP-8642B synthesizer
> for the LO, a Mini-Circuits ZAPD-900-5W-S+ splitter to drive two
> MiniCircuits ZX05-C24LH-S+ mixers, followed by the MiniCircuits
> BLP-50+ low pass filters to yield an IF at 10 meters for the HPSDR
> dual Mercury receivers to use.
>
> I could do something similar for my operations at 23 cm EME but I
> happen to use one of DownEast Microwave's 1296/28 MHz transverters
> there, and it works just fine too.
>
> There are many ways to down convert from the VHF and microwave bands,
> of course!
>
> Good luck with your project!
>
> 73,  Joe K5SO
>
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